Method for controlling an rf generator

ABSTRACT

An RF generator and a method of controlling same includes an RF source; a DC source; and an RF amplifier comprising an RF input, a DC input, and an RF output, the RF amplifier configured to receive an RF signal at the RF input, receive a DC voltage at the DC input, and provide an output power at the RF output; a control unit operably coupled to the DC source and RF source, the control unit configured to receive a power setpoint for the RF output, determine a power dissipation at the RF generator, alter the DC voltage, and repeat the alteration of the DC voltage until determining a final DC voltage that decreases the power dissipation at the RF generator while enabling the output power at the RF output to be equal to or greater than the power setpoint.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/987,718, filed May 2, 2014, the disclosure ofwhich is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The field of the present invention relates to systems and methods forcontrolling an RF generator, including systems for controlling an RFgenerator used in semiconductor plasma processing.

BACKGROUND OF THE INVENTION

Radio frequency (“RF”) generators are used in many applications,including telecommunication, broadcast, and industrial processing. An RFgenerator can be a closed loop system comprising of an RF amplifier, aDC power source, and associated closed loop circuitry.

A block diagram of a typical RF amplifier is shown in FIG. 1. The RFamplifier can receive an RF signal at its RF input and a DC voltage atits DC input. Further, the RF amplifier can output an RF power at its RFoutput. The RF amplifier uses the RF signal to modulate the powerreceived at the DC input to provide an RF power that is higher than thepower at the RF input.

The efficiency of the RF amplifier is dependent upon several factors,including the value of the load connected to its output. As that loadchanges, so does the efficiency of the RF amplifier. The powerdissipation of the RF amplifier (sometimes referred to herein as“PDISS”) is generally understood as the difference between the RF outputpower and the DC input power or, more specifically, the power at the RFoutput minus the power reflected back to the RF amplified and the powerat the DC input.

This power loss (P_(dissipated)) is dissipated as heat among thedifferent components of the RF amplifier. Any heat generated in thecomponents has a direct impact on the reliability of the components. Asa result, in many applications, the RF amplifier is provided withprotection schemes to protect the RF amplifier under conditions such ashigh dissipation. In most cases, the protection schemes are designed tolimit the RF output power and, as a result, limit the DC input power.

While the protection schemes built into RF generators allow the RFgenerator to protect itself, the protection schemes also limit the RFoutput power. Limited RF output power can be problematic for systemsthat utilize RF generators, such as systems providing semiconductorplasma processing. In such a system, an RF generator is supplying powerto enable semiconductor processing. Plasma processing involvesenergizing a gas mixture by imparting energy to the gas molecules byintroducing RF energy into the gas mixture. This gas mixture istypically contained in a vacuum chamber (the plasma chamber), and the RFenergy is typically introduced into the plasma chamber throughelectrodes. If the RF output power is decreased by the generator'sprotection schemes, the power delivered to the plasma chamber isreduced, thereby reducing the process yield for the semiconductorprocessing system. Further, certain plasma conditions may regularlypresent load conditions to the RF generator such that the RF amplifier'sprotection schemes are regularly enabled, thereby affecting the abilityof the semiconductor to be processed.

Thus, there is need for an RF generator and a method for controlling anRF generator that enables the RF generator to operate more efficientlyand/or provide sufficient RF output power.

SUMMARY OF THE INVENTION

The present invention is directed toward systems and methods forcontrolling an RF generator. Such systems and methods can be used insemiconductor processing, as well as in other applications.

In a first aspect of the present invention, an RF generator includes anRF source configured to provide an RF signal; a DC source configured toprovide a DC voltage; an RF amplifier comprising an RF input, a DCinput, and an RF output, the RF amplifier configured to: receive the RFsignal at the RF input; receive the DC voltage at the DC input; andprovide an output power at the RF output; a control unit operablycoupled to the DC source and RF source, the control unit configured to:receive a power setpoint for the RF output; determine a powerdissipation at the RF generator; alter the DC voltage; and repeat thealteration of the DC voltage until determining a final DC voltage thatdecreases the power dissipation at the RF generator while enabling theoutput power at the RF output to be equal to or greater than the powersetpoint.

In a second aspect of the present invention, a method for controlling anRF generator includes a) providing an RF amplifier, the RF amplifiercomprising a DC input, an RF input, and an RF output, the RF amplifierconfigured to provide an output power at the RF output; b) providing anRF signal to the RF input of the RF amplifier; c) providing a DC voltageto the DC input of the RF amplifier; d) receiving a power setpoint forthe RF output; e) determining a power dissipation at the RF generator;f) altering the DC voltage; g) repeating step f) until determining afinal DC voltage that decreases the power dissipation at the RFgenerator while enabling the output power at the RF output to be equalto or greater than the power setpoint.

In a third aspect of the present invention, a method of fabricating asemiconductor includes placing a substrate in a plasma chamberconfigured to deposit a material layer onto the substrate or etch amaterial layer from the substrate; and energizing plasma within theplasma chamber by coupling RF power from an RF source into the plasmachamber to perform a deposition or etching; wherein the RF power isprovided by an RF generator, the RF generator controlled by: a)providing an RF amplifier, the RF amplifier comprising a DC input, an RFinput, and an RF output, the RF amplifier configured to provide anoutput power at the RF output; b) providing an RF signal to the RF inputof the RF amplifier; c) providing a DC voltage to the DC input of the RFamplifier; d) receiving a power setpoint for the RF output; e)determining a power dissipation at the RF generator; f) altering the DCvoltage; g) repeating step f) until determining a final DC voltage thatdecreases the power dissipation at the RF generator while enabling theoutput power at the RF output to be equal to or greater than the powersetpoint.

Accordingly, an improved RF generator, along with systems and methodsincorporating same, is disclosed. Advantages of the improvements will beapparent from the drawings and the description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe exemplary embodiments, will be better understood when read inconjunction with the appended drawings. It should be understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown in the following figures:

FIG. 1 is a block diagram of a prior art RF amplifier.

FIG. 2 is a block diagram of an embodiment of a semiconductor processingsystem.

FIG. 3 is a block diagram of an embodiment of an RF generator.

FIG. 4 is a flow chart of an embodiment of a control algorithm for an RFgenerator.

DETAILED DESCRIPTION OF THE INVENTION

The description of illustrative embodiments according to principles ofthe present invention is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments of the inventiondisclosed herein, where circuits are shown and described, one of skillin the art will recognize that for the sake of clarity, not alldesirable or useful peripheral circuits and/or components are shown inthe figures or described in the description. Moreover, the features andbenefits of the invention are illustrated by reference to the disclosedembodiments. Accordingly, the invention expressly should not be limitedto such disclosed embodiments illustrating some possible non-limitingcombinations of features that may exist alone or in other combinationsof features; the scope of the invention being defined by the claimsappended hereto.

As used throughout, ranges are used as shorthand for describing each andevery value that is within the range. Any value within the range can beselected as the terminus of the range. In addition, all references citedherein are hereby incorporated by reference in their entireties. In theevent of a conflict in a definition in the present disclosure and thatof a cited reference, the present disclosure controls.

The method described herein controls an RF generator by adjusting the DCvoltage (sometimes referred to as the DC rail) presented to the RFamplifier such that the RF amplifier can operate in a high efficiencymode. A control algorithm can enable the RF output power to reach adesired power (referred to herein as the power setpoint) and can alterthe DC voltage to provide a comparable output power while minimizingpower dissipation. This method will be described in greater detailbelow.

Referring to FIG. 2, a semiconductor device processing system 5utilizing an RF generator 10 is shown. The system 5 includes an RFgenerator 10, a matching network 20, and a plasma chamber 30. Thesemiconductor device can be a microprocessor, a memory chip, or othertype of integrated circuit or device. A substrate 40 can be placed inthe plasma chamber 30, where the plasma chamber 30 is configured todeposit a material layer onto the substrate 40 or etch a material layerfrom the substrate 40. Plasma processing involves energizing a gasmixture by imparting energy to the gas molecules by introducing RFenergy into the gas mixture. This gas mixture is typically contained ina vacuum chamber (the plasma chamber 30), and the RF energy is typicallyintroduced into the plasma chamber 30 through electrodes. Thus, theplasma can be energized by coupling RF power from an RF source 105 intothe plasma chamber 30 to perform deposition or etching.

In a typical plasma process, the RF generator 10 generates power at aradio frequency—which is typically within the range of 3 kHz and 300GHz—and this power is transmitted through RF cables and networks to theplasma chamber 30. In order to provide efficient transfer of power fromthe RF generator 10 to the plasma chamber 30, an intermediary circuit isused to match the fixed impedance of the RF generator 10 with thevariable impedance of the plasma chamber 30. Such an intermediarycircuit is commonly referred to as an RF impedance matching network, ormore simply as an RF matching network. The purpose of the RF matchingnetwork 20 is to transform the variable plasma impedance to a value thatmore closely matches the fixed impedance of the RF generator 10.Commonly owned U.S. patent application Ser. No. 14/669,568, thedisclosure of which is incorporated herein by reference in its entirety,provides an example of such a matching network.

The semiconductor device processing system 5 is an example of a systemthat can utilize the RF generator 10. The RF generator 10, however, isnot so limited, as it could be used in a variety of other applicationsthat require RF energy. Such systems can include systems fortelecommunication, broadcast, and industrial processing.

Referring now to FIG. 3, a block diagram of an embodiment of an RFgenerator 10 is shown. The RF generator 10 includes an RF amplifier 100having an RF input 110, a DC input 130, and an RF output 120. An RFsource 105 provides an RF signal to the RF amplifier 100 at the RF input110. A DC source 140 provides a DC voltage to the RF amplifier 100 atthe DC input 130. The RF signal can modulate the power received at theDC input 130 to provide an RF output power at the RF output 120 that ishigher than the power at the RF input 110. The RF source can be anydevice capable of providing a sufficient RF signal for operation of anRF generator, and the DC source can be any device capable of providing asufficient DC signal for operation of an RF generator.

A sensor 160 is connected to the RF output 120. The sensor 160 isconfigured to detect an RF output parameter. The RF output parameter canbe any parameter (or parameters) measurable at the RF output 120,including a voltage, a current, and a phase angle between the voltageand current. In the exemplified embodiment, the sensor 160 detects thevoltage, the current, and the phase angle between the voltage and thecurrent at the RF output 120.

Another sensor 170 is connected to the DC source 140. This sensor 170 isconfigured to detect a DC input parameter. The DC input parameter can beany parameter (or parameters) measurable at the DC input 130, includinga voltage, a current, and a phase angle between the voltage and current.

The RF generator 10 further includes a control unit 150 that can becoupled to the RF source 105, the DC source 140, and the sensors 160,170 of the RF generator 10. The control unit 150 can provide severalfunctions for the RF generator 10. The control unit 150 can receiveinstructions from a user or a system at an input 151. The control unit150 can receive the RF output parameter from sensor 160 and determinethe RF output power. Further, the control unit 150 can receive the DCinput parameter from sensor 170 and determine the DC input power.

Further, the control unit 150 can generate and transmit instructions toother components of the system 5. The control unit 150 can sendinstructions to the DC source 140 to alter the DC voltage provided tothe RF amplifier 100. Further, the control unit 150 can sendinstructions to the RF source 105 to alter the RF signal provided to theRF amplifier 100. Instruction to the RF source 105 can be sent as a PDACsignal. The PDAC signal (or “PDAC”) can be any signal sent by thecontrol unit 150 to the RF source 105 to alter the RF signal output ofthe RF source 105. In the preferred embodiment, the PDAC is a DC signalthat alters the amplitude of the RF signal. The PDAC can increase ordecrease how hard the RF amplifier 100 is working to increase the RFoutput power. The control unit 150 can be programmed to know the properPDAC value to send to produce the desired result for the RF amplifier100.

The control unit 150 can be programmed to carry out a control algorithmfor determining the instructions to send to the DC source 140 and/or RFsource 105. This algorithm will be discussed in further detail below.

The control unit 150 is configured with an appropriate processor and/orsignal generating circuitry to provide signals for controllingcomponents of the RF generator 10, such as the DC source 140 and RFsource 105. In the exemplified embodiment, the control circuit 150includes a processor. The processor may be any type of properlyprogrammed processing device, such as a computer or microprocessor,configured for executing computer program instructions (e.g. code). Theprocessor may be embodied in computer and/or server hardware of anysuitable type (e.g. desktop, laptop, notebook, tablets, cellular phones,etc.) and may include all the usual ancillary components necessary toform a functional data processing device including without limitation abus, software and data storage such as volatile and non-volatile memory,input/output devices, graphical user interfaces (GUIs), removable datastorage, and wired and/or wireless communication interface devicesincluding Wi-Fi, Bluetooth, LAN, etc. The processor of the exemplifiedembodiment is configured with specific algorithms to enable the RFgenerator 10 to operate as described herein.

Referring now to FIG. 4, a flow chart of an embodiment of a controlalgorithm for an RF generator 10 is shown. It should be noted at theoutset that the exemplified control algorithm (sometimes referred toherein simply as the “process”) contains several routines, some of whichcan run independently of other routines. For example, the steps forensuring maximum efficiency can be run independently of the steps forachieving an RF output power corresponding to the setpoint. In theexemplified embodiment, the control algorithm includes steps forachieving the power setpoint and steps for ensuring maximum efficiencyat that setpoint. In alternative embodiments, the achievement of thepower setpoint can be assumed and the control algorithm can refer simplyto the steps for ensuring maximum efficiency at that setpoint. In yetother embodiments, the control algorithm can simply provide a processfor achievement of the power setpoint. The exemplified embodiment isjust one approach for carrying out the invention.

Table 1 below provides certain abbreviations used in the flow chart.

TABLE 1 Abbreviation Meaning CALG Control algorithm DCSET Startup DCsetpoint DCSETP DC setpoint based on power setpoint (SETP) PSETP Powersetpoint DCMAX Maximum DC voltage provided by DC source DCMIN Minimum DCvoltage provided by DC source DCSTEP Predetermined amount by which DCvoltage is increased or decreased PDAC Power signal to the RF sourcePDACL Limit on PDAC (maximum PDAC) MAXEFF Maximum efficiency mode PDISSPower dissipation of the amplifier

The exemplified process for controlling the RF generator 10 allows theuser to select whether to operate the RF generator 10 in maximumefficiency (MAXEFF) mode. The maximum efficiency option can becontrolled by the user at the control unit 150 by a switch or by anyother known method for enabling a process. In alternative embodiments,the maximum efficiency steps can always be enabled when the DC source140 is turned ON.

The process also allows a user to set a maximum DC voltage provided byDC source (DCMAX), a minimum DC voltage provided by DC source 140, and astartup DC setpoint (DCSET). The startup DC setpoint (DCSET) is theinitial DC voltage provided by the DC source 140 when the DC source 140is turned ON. In alternative embodiments, one or more of the DCMAX,DCMIN, and DCSET can be fixed values, or can be determined by a program.

The exemplified process for controlling the RF generator 10 begins bythe DC source 140 being turned ON (step 202). In the exemplifiedembodiment, the DC source 140 receives power from an AC power source anda switch enables a user to turn the DC source 140 ON. When the DC source140 is initially turned ON, it provides the startup DC setpoint (DCSET).In other embodiments, the step of providing a startup DC setpoint can beomitted.

Next, a desired power at the RF output (PSETP) is received and, inresponse, a DC voltage (DCSETP) is provided (step 204). In this step,the control unit 150 can receive an instruction to have the RF generator10 provide a specific RF output power. This desired RF output power isreferred to as the power setpoint (PSETP). The power setpoint can bereceived from another system (e.g., a semiconductor processing system),a user input, or any other source. In response to the requested powersetpoint, the control unit 150 can instruct the DC source 140 to providea DC voltage (DC setpoint (DCSETP)) likely to result in the desiredpower setpoint. The control unit 150 can be programmed in advance toinstruct certain DC setpoints in response to certain received powersetpoints. For example, a table of power setpoints can be provided alongwith corresponding DC setpoints.

Next, the control unit 150 calculates the power dissipated by the RFamplifier 100 (PDISS), and stores this value (Old PDISS) (step 206). Asstated above, the power dissipated (PDISS) can be calculated as follows:

P _(dissipated) =P _(RF output) −P _(reflected) −P _(DC input)

By sensor 160 and the RF output parameters measured, the control unit150 can determine the RF output power (P_(RF output)) and the powerreflected (P_(reflected)). In the exemplified embodiment, the sensor 160is a power sensor that measures voltage, current, and the phase anglebetween them at the RF output 120. In alternative embodiments, the powersensor can be a directional coupler that couples signals representativeof forward and reflected power from the main power path, or can beanother type of sensor. By sensor 170 and the DC input parametersmeasured, the control unit 150 can determine the power at the DC input130. Using this information, the control unit 150 can determine thepower dissipated by the RF amplifier 100 (P_(dissipated) or PDISS). Thecontrol unit 150 can then store this value (Old PDISS) in memory (notshown) for future use.

Next, the process determines whether the control algorithm (CALG) isturned ON (step 208). The control algorithm can be controlled by theuser at the control unit 150 by a switch or by any other known methodfor enabling an algorithm. In alternative embodiments, the controlalgorithm can always be ON when the DC source 140 is turned ON. If thecontrol algorithm is not turned ON, then the DC source 140 will simplyprovide the DC voltage of the DC setpoint, as discussed above.

Next, the process determines whether the RF generator 10 can make thepredetermined power setpoint (PSETP) (step 210). This step can becarried out by the sensor 160 determining the RF output parameter andcommunicating this parameter to the control unit 150. The control unit150 can then be programmed to determine the RF output power and whetherit corresponds with the power setpoint. As used in this step, the term“make” refers to whether the RF output power can equal the powersetpoint. The term make can also refer to exceeding the power setpoint,though such an occurrence is unlikely in such a system.

If the RF generator 10 can make the PSETP, the process next determineswhether the maximum efficiency option (MAXEFF) has been turned ON (step212). If the maximum efficiency option (a further capability of thecontrol algorithm) is not turned ON, then the process will determinewhether the PDAC is greater than or equal to the PDACL (step 214). ThePDAC, discussed above, is sent by the control unit 150 to the RF source105 and helps control how hard the RF amplifier 100 is working toproduce the desired RF output power. The PDACL is a predetermined limiton how hard the RF amplifier 100 can be pushed.

If the PDAC has exceeded the PDACL, the process increases the DC voltageby a predetermined amount (DCSTEP) (step 216). This can be carried outby the control unit 150 sending such instructions to the DC source 140.The increase of the DC voltage helps to ease the burden on the RFamplifier 100, thereby decreasing the PDAC. The process then againdetermines whether the generator 10 can make the power setpoint (step210) and again determines whether the PDAC is greater than or equal toPDACL (step 214). This process repeats until the PDAC is less than thePDACL. In alternative embodiments, the process can stop when the PDAC isless than or equal to the PDACL.

Once the PDAC is less than the PDACL, the process goes to point A, whichrequires determination of whether the power setpoint (PSETP) changed(step 218). The power setpoint can change for a variety of reasons. Forexample, in a system 5 for the plasma processing of semiconductors, thesystem 5 will require different RF output powers at different stages ofthe processing. If the power setpoint has changed, the process returnsto step 204. If not, the process returns to step 208.

Returning to the maximum efficiency option (MAXEFF), if this option isset to ON, the process again calculates and stores the power dissipationat the RF amplifier 100 (PDISS) (step 220). This calculation is carriedout in a manner similar to that discussed with regard to step 206.

The process then determines whether the New PDISS (calculated in step220) is less than the Old PDISS (calculated in step 206) (step 222) atthe current voltage. The current voltage is sometimes referred to as the“intermediate voltage” if it is a voltage different from the initialvoltage (DCSETP) and the final voltage. This step can be carried out bythe control unit 150. Several factors can cause the PDISS to change,such as a change to the load. If the New PDISS is less than the OldPDISS, then the power dissipation is increasing, and therefore theefficiency of the RF generator 10 is decreasing.

If it is determined that the New PDISS is not less than the Old PDISS(the New PDISS is greater than or equal to the Old PDISS), and thereforethe PDISS is increasing, the process stops changing the DC voltage andbecomes the final voltage. This step of the exemplified embodiment canenable the power dissipation to be a substantially minimum powerdissipation (and therefore maximum efficiency) at which the output poweris equal to the predetermined power setpoint.

The process then returns to point A and step 218 of the process (step224). At those points in the exemplified embodiment when the processstops changing the DC voltage and returns to point A, the voltage isconsidered set at the final voltage. The final voltage is final in thesense that it is the DC voltage at which the DC source 140 remains untilthe power setpoint (PSETP) or some other factor changes prompting areassessment of the DC voltage and its effects, as occurs in step 218.The term “final” does not mean that the voltage is permanent or cannotchange. Note further that when the process determines whether a value is“less than” or “greater than” another value, in alternative embodimentsthis determination can be replaced with a determination of whether avalue is “less than or equal to” or “greater than or equal to,”respectively. Similarly, in alternative embodiments, “less than or equalto” and “greater than or equal to,” can be replaced with “less than” and“greater than,” respectively.

If it is determined that the New PDISS is less than the Old PDISS, theprocess determines whether the DC voltage is at its minimum (DCMIN)(step 226). If it is, then the process stops changing the DC voltage andreturns to point A and step 218 of the process (step 228).

If the DC voltage is not at its minimum (DCMIN), the process determineswhether the DC voltage was increased at its most recent change (230).The control unit 150 can carry out this determination, where previouschanges to the DC voltage are stored in a memory (not shown) connectedto or part of the control unit 150.

If it is determined that the DC voltage was increased at its most recentchange (230), then the process stops changing the DC voltage and returnsto point A and step 218 of the process (step 232). If it is determinedthat the DC voltage was not increased at its most recent change (230),then the DC voltage is decreased by DCSTEP (step 234). The process thenreturns to step 202 and determining whether the RF generator 10 can makethe power setpoint at this newly decreased DC voltage. These steps ofthe exemplified embodiment enable the process to determine asubstantially minimum DC voltage at which the output power is equal tothe predetermined power setpoint.

If the RF generator 10 cannot make the power setpoint (PSETP), theprocess determines whether the RF amplifier's protection schemes havebeen enabled (step 236). For example, a protection scheme can limit thevoltage on the drain of a field-effect transistor (FET) in the generator10. The voltage on the FET drain can be measured. If the measured drainvoltage exceeds a predetermined value, the protection scheme can lowerthe RF output power to lower the drain voltage. This can prevent thegenerator 10 from failing, but can also reduce the RF output power belowthe requested power setpoint.

If it is determined that the RF amplifier's protection schemes have beenenabled, the process proceeds to step 226 and determines whether the DCvoltage is at DC minimum. If it is determined that the RF amplifier'sprotection schemes have not been enabled, the process determines whetherthe PDAC is at its limit (PDACL), similar to step 214 (step 238). If itis not, the process returns to point A and step 218 of the process.

If the PDAC is at its limit, the process determines whether the DCvoltage is at its maximum (DCMAX) (step 240). If it is, then the processstops changing the DC voltage and returns to point A and step 218 of theprocess (step 242). If the PDAC is not at its limit, the processincreases the DC voltage by DCSTEP (step 244) and then returns to step210 to determine whether the RF generator 10 can still make the powersetpoint.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques. It is tobe understood that other embodiments may be utilized and structural andfunctional modifications may be made without departing from the scope ofthe present invention. Thus, the spirit and scope of the inventionshould be construed broadly as set forth in the appended claims.

What is claimed is:
 1. An RF generator comprising: an RF amplifiercomprising an RF input, a DC input, and an RF output, the RF amplifierconfigured to: receive at the RF input an RF signal from an RF source;receive at the DC input a DC voltage from a DC source; and provide anoutput power at the RF output; a control unit operably coupled to the DCsource and the RF source, the control unit configured to: receive apower setpoint for the RF output; determine a power dissipation at theRF generator; alter the DC voltage; and repeat the alteration of the DCvoltage until determining a final DC voltage that decreases the powerdissipation at the RF generator while enabling the output power at theRF output to be equal to or greater than the power setpoint.
 2. The RFgenerator of claim 1 wherein the final DC voltage cannot be less than apredetermined minimum DC voltage value.
 3. The RF generator of claim 2wherein the final DC voltage cannot exceed a predetermined maximum DCvoltage value.
 4. The RF generator of claim 1 wherein the determinationof the final DC voltage includes a determination that, at anintermediate DC voltage, the power dissipation is increasing.
 5. The RFgenerator of claim 4 wherein the altering of the DC voltage comprisesone of (i) reducing the DC voltage by a predetermined amount and (ii)increasing the DC voltage by the predetermined amount.
 6. The RFgenerator of claim 1 wherein the final DC voltage is substantially aminimum DC voltage at which the output power is equal to the powersetpoint.
 7. The RF generator of claim 1 wherein the decreased powerdissipation is substantially a minimum power dissipation at which theoutput power is equal to the power setpoint.
 8. The RF generator ofclaim 7 wherein the power dissipation is the power at the RF outputminus a power at the DC input and a power reflected back to the RFamplifier.
 9. The RF generator of claim 1 wherein the power setpoint isreceived from a semiconductor processing system.
 10. A method ofcontrolling an RF generator, the method comprising: a) providing an RFamplifier, the RF amplifier comprising a DC input, an RF input, and anRF output, the RF amplifier configured to provide an output power at theRF output; b) receiving an RF signal to the RF input of the RFamplifier; c) receiving a DC voltage to the DC input of the RFamplifier; d) receiving a power setpoint for the RF output; e)determining a power dissipation at the RF generator; f) altering the DCvoltage; g) repeating step f) until determining a final DC voltage thatdecreases the power dissipation at the RF generator while enabling theoutput power at the RF output to be equal to or greater than the powersetpoint.
 11. The method of claim 10 wherein the final DC voltage cannotbe less than a predetermined minimum DC voltage value.
 12. The method ofclaim 11 wherein the final DC voltage cannot exceed a predeterminedmaximum DC voltage value.
 13. The method of claim 10 wherein thedetermination of the final DC voltage includes a determination that, atan intermediate DC voltage, the power dissipation is increasing.
 14. Themethod of claim 10 wherein the altering of the DC voltage comprises oneof (i) reducing the DC voltage by a predetermined amount and (ii)increasing the DC voltage by the predetermined amount.
 15. The method ofclaim 10 wherein the final DC voltage is substantially a minimum DCvoltage at which the output power is equal to the power setpoint. 16.The method of claim 10 wherein the decreased power dissipation issubstantially a minimum power dissipation at which the output power isequal to the power setpoint.
 17. The method of claim 16 wherein thepower dissipation is the power at the RF output minus a power at the DCinput and a power reflected back to the RF amplifier.
 18. The method ofclaim 10 wherein the power setpoint is received from a semiconductorprocessing system.
 19. A method of fabricating a semiconductorcomprising: placing a substrate in a plasma chamber configured todeposit a material layer onto the substrate or etch a material layerfrom the substrate; and energizing plasma within the plasma chamber bycoupling RF power from an RF source into the plasma chamber to perform adeposition or etching; wherein the RF power is provided by an RFgenerator, the RF generator controlled by: a) providing an RF amplifier,the RF amplifier comprising a DC input, an RF input, and an RF output,the RF amplifier configured to provide an output power at the RF output;b) providing an RF signal to the RF input of the RF amplifier; c)providing a DC voltage to the DC input of the RF amplifier; d) receivinga power setpoint for the RF output; e) determining a power dissipationat the RF generator; f) altering the DC voltage; and g) repeating stepf) until determining a final DC voltage that decreases the powerdissipation at the RF generator while enabling the output power at theRF output to be equal to or greater than the power setpoint.
 20. Themethod of claim 19 wherein: the final DC voltage cannot be less than apredetermined minimum DC voltage value; the final DC voltage cannotexceed a predetermined maximum DC voltage value; the determination ofthe final DC voltage includes a determination that, at an intermediateDC voltage, the power dissipation is increasing; the altering of the DCvoltage comprises one of (i) reducing the DC voltage by a predeterminedamount and (ii) increasing the DC voltage by the predetermined amount;and the decreased power dissipation is substantially a minimum powerdissipation at which the output power is equal to the power setpoint.